Much like other industries, oil exploration continues to push current boundaries for application of high-tech communications. This is true for transmission of telemetry and control data between electronics located in a borehole and control stations. However, new technology must meet the stringent safety requirements present in most exploratory settings. The concern with safe operating procedures, for example, on an offshore oil rig 102 (FIG. 1) extends to all aspects of rig operation, including data transmission across rig cabling 112 and 118, typically many hundreds of feet in length. In some cases, safety protection is designed internal to certain surface equipment 108, located within the hazardous area, such that special cabling 112 can be used without concern for errant operation of equipment within the safe area 114.
However, for some equipment energy consumption restrictions are required. In these cases safe devices have been used which have very low power ratings. An example of such safe devices are called intrinsically safe, “IS”, devices, certified by various standards agencies, such as Underwriters Labs in the United States and CENELEC in Europe. These IS devices, for example, are certified to have a low power rating, low energy storage capabilities, and low inductance and capacitance ratings. The primary concept for an IS device is that the device be incapable of creating a spark of sufficient energy to ignite a specified hazardous gas. To insure operational integrity of these safe devices, care must be taken in communicating or otherwise coupling to outside devices, located in the safe area, that are not safety certified to the same requirements.
Presently, safety barriers 110 are used for rig cabling 118 transmitting to or from an intrinsically safe certified device located in a hazardous area, such as pressure sensor 120. These safety barriers 110 are generally one or two channel devices which pass electrical signals but limit the transfer of energy to a level that cannot ignite explosive atmospheres. The safety barriers protect hazardous-area wiring and equipment from faults occurring in a safe area, such as control house 114. This allows use of standard, off-the-shelf, safe-area equipment that requires no additional certification.
Currently, information is transferred from the safe device 120, for example a mud pulse pressure sensor, across some length of cable 118 from a hazardous area through the safety barrier 110 to the control house 114. The signal is typically transmitted in analog form. Due to the high noise environment of a rig, for example, and the sometimes very long length of cable across which the signal must be transmitted, circuitry at the sensor system applies some level of gain to the signal before transmitting through the safety barrier. However, current applications are pushing the edge of this technology. Further, with ever increasing demand for complete and instantaneous information by the rig operator, for example, these pure analog systems are being fitted with analog-to-digital converter circuitry.
FIG. 2 illustrates a common application for operation of an IS sensor 200. For example, safe sensor 200 includes a pressure diaphragm 206 to sense pressure pulses transmitted via standpipe 222 in mud-pulse telemetry systems. As telemetry is detected an analog signal from IS sensor 200 is transmitted out of the hazardous zone 202 to an ADC circuit 218 in a safe zone 204. Here, the safety barrier 214 is placed between the zones, but not before the signal 210 is subjected to numerous interference sources 212, such as radios, high power rig motors and SCR power noise, to name a few. Assuming the signal is not degraded significantly, the cpu 220, located in a control house 224, receives a digital version of the analog signal in the safe area 204. Once the analog signal has been digitized, its immunity to noise and signal degradation across a length of cable increases substantially. However, as noted, the analog signal is still subjected to many interference sources 212 and sometimes hundreds of feet of cabling which can render the received signal unreadable.
Digital conversion electronics have been placed within the hazardous zone to provide a digitized signal earlier along the transmission line. The digital signals have a number of advantages over analog signals, including increased immunity and the ability to transmit at higher data rates. Present systems have achieved data rates of about 1.2K baud using the HART protocol to communicate with devices located in hazardous locations. Higher data rates are desired. One manufacturer has an active barrier that can achieve 19.2K baud using proprietary active barriers. However, the combination of the capacitance of very long cabling and the resistance of present safety barriers causes signal degradation at high data rates, essentially imposing a RC time constant on the digital signal. Furthermore, such active barriers are expensive and add equipment and design cost to the system. Even more prohibiting, as the rate of data transfer increases, the RC effects can override the data being transferred to the point that the received signal cannot be resolved.